Crude oil washing (COW) is a system whereby oil tanks on a tanker are cleaned out between voyages not with water, but with crude oil - the cargo itself. The solvent action of the crude oil makes the cleaning process far more effective than when water is used. (There is usually a final water rinse but the amount of water involved is very low.) The system helps prevent pollution of the seas from operational measures.
Crude oil washing regulations
Crude oil washing was made mandatory for new tankers by the 1978 Protocol to the MARPOL Convention.
MARPOL Annex I Regulation 13 (6) requires every new crude oil tanker of 20,000 tons deadweight and above to be fitted with a cargo tank cleaning system using crude oil washing.
Regulation 13B states that COW installation and arrangements should comply with at least all of the provisions of the Specifications for the Design, Operation and Control of Crude Oil Washing Systems adopted by IMO in 1978, as may be revised.
To perform Crude oil washing in correct and efficient manner every ship must have –
· Efficient and approved COW system and equipment manual
· Skilled Personnel on board who are properly trained to perform the complete operation
Before starting the operation
· Confirm all pre arrival checks are performed
· The complete COW operation to be discussed with ship and shore staff
· Set a communication channel between ship and shore facility for COW operation-Ship shore interface
· Signal and Emergency signs to be discussed to stop the operation between shore and ship staff
· Inert Gas plant to be working and oxygen content must be less than 5 %
· Fixed Oxygen analyser to be checked and calibrated for proper functioning
· Portable oxygen analyser should be made available and checked for proper functioning.
· Oxygen reading in swash bulkhead tanks must be taken from both the sides
· All tanks must be checked for positive inert gas pressure
· Assign duties to all responsible ship staff. One person to be assigned to check the leakage in the pipe line system as soon as the operation starts
· Check all the equipments under COW system for proper functioning
· Check and Set the line and valves for ship to shore under COW system
Fixed washing machines
The tank washing machines for crude oil washing shall be permanently mounted and shall be of a design acceptable to the Administration.
Fixed tank washing machines, originally constructed for water are used for COW. As a general rule any system designed for effective water washing is even more effective with COW.
The main types of washing machines most commonly used for COW are:
1. single nozzle machines These are usually programmable to cover pre-set sectors (capacities up to about 200 m3/h) and are intended for deck mounting, but submerged unprogrammable single nozzle machines are also available.
2. Dual nozzle unprogrammable washing machines
WASH STRATEGY
10.9.1 Top washing
It is recommended to start the top washing, as soon as possible, when about ¾ of the tank is discharged. Top washing is simpler than bottom washing and may be done simultaneously as the discharging takes place in full rate. To start the top washing a bleed off from one of the discharging lines is lead to the cow main line, and from there, to the cow machines being used. Make sure that the recommended pressure for cow lines and cow machines is kept during the operation.
10.9.2 Bottom washing
Usually bottom washing starts with a small amount of cargo in the tank. The amount in the tank is checked before the bottom washing starts. If sounding (dipping) is used for the check,remember to use a sounding (dipping) rod and a line of material, which does not collect static electricity. It is not possible to sound when the crude oil washing is in process. If so, check the manometer on the suction side of the stripping system.
Make sure that the bottom-washing cycle overlaps the top-washing cycle. In the washing process, each tank depends on the tank construction, the oil’s composition, time since last washing, the equipment’s function and the skill of the operator. If the stripping is internal, it is extremely important to keep an accurate record of the level at the receiving tank. It is recommended to strip directly ashore if the ship is fitted with equipment to do so (vacuum strip). This saves a lot of time and also avoids moving the washing liquid and residues from tank to tank internally.
By the end of the bottom washing, remember that liquid will continue to flow toward the suction for some time. Therefore, some final stripping is necessary. When the bottom washing is completed, remember to properly shut all of the cow machine’s delivery valves.
Crude oil washing regulations
Crude oil washing was made mandatory for new tankers by the 1978 Protocol to the MARPOL Convention.
MARPOL Annex I Regulation 13 (6) requires every new crude oil tanker of 20,000 tons deadweight and above to be fitted with a cargo tank cleaning system using crude oil washing.
Regulation 13B states that COW installation and arrangements should comply with at least all of the provisions of the Specifications for the Design, Operation and Control of Crude Oil Washing Systems adopted by IMO in 1978, as may be revised.
To perform Crude oil washing in correct and efficient manner every ship must have –
· Efficient and approved COW system and equipment manual
· Skilled Personnel on board who are properly trained to perform the complete operation
Before starting the operation
· Confirm all pre arrival checks are performed
· The complete COW operation to be discussed with ship and shore staff
· Set a communication channel between ship and shore facility for COW operation-Ship shore interface
· Signal and Emergency signs to be discussed to stop the operation between shore and ship staff
· Inert Gas plant to be working and oxygen content must be less than 5 %
· Fixed Oxygen analyser to be checked and calibrated for proper functioning
· Portable oxygen analyser should be made available and checked for proper functioning.
· Oxygen reading in swash bulkhead tanks must be taken from both the sides
· All tanks must be checked for positive inert gas pressure
· Assign duties to all responsible ship staff. One person to be assigned to check the leakage in the pipe line system as soon as the operation starts
· Check all the equipments under COW system for proper functioning
· Check and Set the line and valves for ship to shore under COW system
Fixed washing machines
The tank washing machines for crude oil washing shall be permanently mounted and shall be of a design acceptable to the Administration.
Fixed tank washing machines, originally constructed for water are used for COW. As a general rule any system designed for effective water washing is even more effective with COW.
The main types of washing machines most commonly used for COW are:
1. single nozzle machines These are usually programmable to cover pre-set sectors (capacities up to about 200 m3/h) and are intended for deck mounting, but submerged unprogrammable single nozzle machines are also available.
2. Dual nozzle unprogrammable washing machines
WASH STRATEGY
10.9.1 Top washing
It is recommended to start the top washing, as soon as possible, when about ¾ of the tank is discharged. Top washing is simpler than bottom washing and may be done simultaneously as the discharging takes place in full rate. To start the top washing a bleed off from one of the discharging lines is lead to the cow main line, and from there, to the cow machines being used. Make sure that the recommended pressure for cow lines and cow machines is kept during the operation.
10.9.2 Bottom washing
Usually bottom washing starts with a small amount of cargo in the tank. The amount in the tank is checked before the bottom washing starts. If sounding (dipping) is used for the check,remember to use a sounding (dipping) rod and a line of material, which does not collect static electricity. It is not possible to sound when the crude oil washing is in process. If so, check the manometer on the suction side of the stripping system.
Make sure that the bottom-washing cycle overlaps the top-washing cycle. In the washing process, each tank depends on the tank construction, the oil’s composition, time since last washing, the equipment’s function and the skill of the operator. If the stripping is internal, it is extremely important to keep an accurate record of the level at the receiving tank. It is recommended to strip directly ashore if the ship is fitted with equipment to do so (vacuum strip). This saves a lot of time and also avoids moving the washing liquid and residues from tank to tank internally.
By the end of the bottom washing, remember that liquid will continue to flow toward the suction for some time. Therefore, some final stripping is necessary. When the bottom washing is completed, remember to properly shut all of the cow machine’s delivery valves.
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